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I had very similar issues on my boat as well. Replaced the RF head unit, both remotes and everything in-between. Replaced head unit a second time and problem went away. First replacement was a refurbished head unit.
I would say it is almost definitely a head unit issue. Probably why Cobalt has moved away from RF units in favor of Garmin Fusion, someone correct me if I am wrong on this.

I just ordered a Rockford Fosgate head unit from Creative Audio (good friends of members of this forum), and managed to get a couple bum units in a row. The first one didn't power on at all (it just blew fuses), and the second one had a dead USB port. After a conversation with Rockford Fosgate tech support and then with James Taylor at Creative Audio, I switched to a Fusion RA-670 head unit and ERX400 remote.

Rockford Fosgate also told me on the phone during troubleshooting that the PMX-5CAN unit would only support up to 999 songs on a flash drive that could be no bigger than 32GB, so that also tipped the scaled toward Fusion. Customer support from Creative Audio and Rockford Fosgate were both top notch.

jhnmdahl wrote: Wed Jun 21, 2023 11:00 am Rockford Fosgate also told me on the phone during troubleshooting that the PMX-5CAN unit would only support up to 999 songs on a flash drive that could be no bigger than 32GB, so that also tipped the scaled toward Fusion.

Does that mean my Apple phone will not bluetooth to my RF system if I have 1000 songs downloaded?

Krob, that should work fine because your Apple phone is only streaming the one song you choose on the iPhone to the RF head unit. What doesn't work is putting 500 albums on a USB thumb drive and connecting it via USB (which is how we play a lot of our music).

Very interested to know what you think of that fusion unit.